Preheat oven to 375°F. Grease a 2 1/2-quart shallow ceramic or glass baking dish.
In a large bowl, combine rhubarb, strawberries and vanilla. In a small bowl, whisk together granulated sugar and cornstarch. Stir into fruit mixture. Pour fruit into prepared baking dish.
In a bowl, combine oats, brown sugar, flour, cinnamon, salt and nutmeg. Using your fingers, pinch butter into dry ingredients until combined. Stir in nuts. Spread topping evenly on fruit.
Place dish on foil-lined baking sheet and bake crisp until topping is golden brown and fruit is bubbling, about 45 minutes. Let cool for 10 minutes. Serve warm with vanilla ice cream.
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